Since the end of 2008 the German Ultrafine Aerosol Network (GUAN), an atmospheric observation network designed for long-term observations, is operational. The measurements are targeted at improving the description of the environmental aerosol (particulate matter), focussing on parameters that are relevant for human exposure but also climate effects. The measurements include sub-pm particle number size distributions, soot mass concentrations, and size-segregated chemical composition at a number of up to eleven observation sites. Specialised campaign measurements are added for a deeper understanding of the properties, sources, and processes related to ultrafine particles and soot. First observations from December 2008 and January 2009 illustrate the value of the network when characterising spatial and temporal effects in the atmospheric aerosol over an area such as Germany.